In order to understand how children learn, one must understand not only what facts they know, but also how children structure their knowledge and how they use their knowledge when they reason. The goal of this project is to gain insight into how children and adults learn through an examination of the development of children's biological knowledge. Previous work has examined what children know about individual properties that characterize biological kinds, such as growth, and whether children realize that living things are uniquely affected by biological causes. This project adds to the existing body of literature by examining how children structure and reason with their knowledge about biological properties. Moreover, the experiments will provide information about how the structure and reasoning processes change with age. The goal of Experiment l is to examine how children structure their knowledge about living things; e.g., do children realize that things that grow also reproduce, or do they look at these attributes as unrelated facts? When children learn about living things, they must organize their knowledge about the categories and master several overlapping distinctions. Children must distinguish between animals, plants, and nonliving things; living things and nonliving things; and animates and inanimates. Children must learn to which group various properties apply, as well as when and how the distinctions are relevant. Through a "hidden picture" procedure, Experiment l provides insight into the developing structure of children's biological knowledge. An experimenter will ask 4- to 6-year-old children to help figure out some things about a hidden picture. The experimenter will read a given property; for example, "I'm looking at a picture of something that grows," ask each child yes/no questions about the properties the pictured object might or might not possess, then ask the child to identify' the object from a set of pictures of plants, animals, and nonliving things. This procedure will provide insight into the developing structure of children's biological knowledge. The goal of Experiment 2 is to examine whether 4- to 6-year-olds can use the category of "living things" as a basis for reasoning, or whether they are limited to reasoning about plant and animal categories separately. In an oddity task, children are reminded of properties they know to be true of living and nonliving things. Then- children are taught a new property of an animal, plant, or human-made object and asked which of two choices is most likely to share the property. This procedure examines how children use the information they already know to solve problems and how this ability changes. It is necessary to investigate how new information is integrated with existing knowledge in order to understand how human beings learn. This project examines the development of children's reasoning in the domain of biology to provide insight into how children build models or theories of the world that allow them to reason in the same way as adults in their culture.